Various geomagnetic application devices have been used that measure a geomagnetic vector and utilize results of the measurement, such as an electronic compass which includes a magnetic sensor for detecting earth magnetism and performs azimuth measurement based on a geomagnetic vector detected with the magnetic sensor and an air mouse which enables operations on a computer screen by changing its own direction in the air. In this case, the electronic compass can calculate its own azimuth based on the data of a geomagnetic vector obtained from the magnetic sensor and use it when deciding a user's traveling direction in navigation for guiding him to his destination on the screen of a cellular phone, etc. The air mouse having a built-in magnetic sensor is rotated in the air to calculate its own direction from the earth magnetism detected during the rotation, and enables an operation on computer screen based on the calculated direction. Further, a magnetic gyroscope can calculate a rotation axis based on detected results of earth magnetism to obtain its own rotation angle and angular speed, therefore, it is very important in this case also to accurately calculate an azimuth, etc. from the detected earth magnetism. As stated above, detection of earth magnetism can be applied for various purposes, the geomagnetic vector is required to be accurately measured for such occasions.
The geomagnetic application devices mainly include mobile devices such as a cellular phone, a portable game machine, and a tablet PC for portable use. Because such a geomagnetic application device incorporates a magnetized electronic component such as a speaker, the magnetic sensor built in the device consequently detect a magnetic field as combined one consisting of earth magnetism and a magnetic field generated by such an electronic component. Therefore, calibration processing is required to correct an error (offset) caused by the magnetic field generated by the built-in electronic component of the geomagnetic application device.
Accordingly, in the past, an offset value was estimated by utilizing postural change of the geomagnetic application device, which was resulted from users' operation of the device for some purpose. And in some cases, the geomagnetic application device was forced to rotate by the user in a specific manner so as to cause postural change of the device and to be measured with the magnetic sensor. Then, an offset value was estimated on the basis of measured data on a magnetic sensor, which had been collected in various postures of the device.
Specifically, if output components of a triaxial magnetic sensor during rotating under constant earth magnetism are respectively plotted in a space, the resultant trajectory forms a sphere. The sphere is referred to as an azimuth sphere, a center point of which corresponds to an offset of the magnetic sensor. Therefore, calibrating the offset is none other than employing the center point of the azimuth sphere obtained based on the earth magnetism measured in the various azimuths and postures as a new offset value. The radius of the azimuth sphere corresponds to the intensity of earth magnetism.
To accurately obtain coordinates of the center point of the azimuth sphere, it is necessary to obtain data of measurement points on the azimuth sphere in an area as wide as possible by operation for turning the geomagnetic application device in every direction evenly.
However, such operations impose excessive burdens on the user. To eliminate the burdens, a technique has been proposed that utilizes a change in posture of the geomagnetic application device, which was resulted from users' operation which the user thinks necessary without purposely requesting the user to conduct a specific operation for turning the device so that the data which satisfies certain conditions adequate to accurately determine the azimuth sphere are selected among the obtained measurement points to obtain the center point of the azimuth sphere (Patent Document 1).